Cellular and tissue mechanisms involved in the pathogenesis of a spontaneously occurring, familial form of retinal dysplasia of English springer spaniel dogs will be studied. Sequential stages in the development of the dysplastic retinal defect will be examined by clinical ophthalmoscopy, electroretinography, ultrastructural histology and ultrastructural cytochemistry. Particular emphasis will be given to the distribution, differentiation and functional competence of choriocapillaris vasculature, Bruch's membrane and pigment epithelium in regions of dysplastic retinal growth.